This invention relates to a fastener, preferably a one-piece plastic fastener such as a rivet or screw grommet. It is well known in the art to provide a hollow fastener having a tapered internal bore passing through the head and shank for accepting an expander element. Such devices are found in the U.S. Pat. Nos. 2,898,798 to Carno; Rapata 3,385,157 and Birmingham 3,411,397. Each of these devices disclose a tapered internal bore in which the shank portions or prongs are merely radially expanded by a drive pin or screw, either separate or integrally molded, which when axially moved engages the tapered wall of the bore and radially expands the free ends of the shank portions or prongs outwardly beyond the surface of the apertured panel opposite the surface engaged by its head or against the side wall of a blind bore.